Ok, this post will be a little long. Sorry about that, but I find diff settings one of the most complex areas of GPL. However, if you can get the hang of it, it makes a really nice difference to how the car behaves, and enables you to control direction with the throttle, as well as the steering.

It's taken me a long time to learn how to set the cars up to best suit my style of driving. The info I post below may not suit your style of driving - we're all different after all. But if you read it, and I make it clear enough to understand, you might pick up some info on how to do your own experiments with the diff settings to find something that will work for you.

Ok, let's get started with answering your question. If you have the power and coast locking set to very similar values, I'm 99% sure you won't notice any difference between 45/50/3 and 55/60/4. Drivers will usually only notice the effect of the diff settings if there's a transition between two different locking values. With both coast and power locking set at 13-14% for both setups, there won't be any noticeable transitions, so the car shouldn't handle any differently under coast or power conditions. The trick is to set the diff locking to different values so that you can do what you want with the car under different circumstances.

For example, when I set my diff values, I usually use greater locking for coast, and lower locking for power. The greater coast locking means the car is more stable under braking, as the rear end is less likely to get squirrely. As mentioned in the "Four-Wheel Drift" book, higher coast locking also acts a little bit like an anti-lock braking system, as it discourages one of the back wheels from locking up on its own under braking, which always puts you into a nasty spin. Although higher coast locking hinders turn-in (because the locked rear axle wants to keep going 'straight'), I usually leave my braking late and continue braking during turn-in (trail-braking), so the extra weight transfer to the front of the car increases front grip and compensates for the locked rear end.

Once I've got the car slowed down, turned in and rotating into the corner, I gently release the brakes and let the weight resettle evenly between front and back. I then give a gentle press on the throttle to unlock the diff to its lower power setting. This will enable the car to turn more easily, so I can more easily control where it's pointing as I pass the apex and start to exit the corner.

If I'm trying to be as fast as possible, I sometimes carry a bit too much speed into the apex, and this becomes apparent when the car starts to understeer a bit during corner exit, and starts sliding to the outside of the track. To compensate for this, I can try giving a brief 'blip' on the throttle, which usually causes the inside rear wheel to lose grip and start spinning. This in turn causes the rear end to lose a bit of grip compared to the front end, and the car goes into a slight oversteer, with the rear end stepping out. This corrects the slide to the outside of the track, and points the nose of the car down the straight. After the quick 'blip', I reduce throttle to allow the spinning rear wheel to grip again, and then I start to reapply power smoothly to begin accelerating out of the corner and down the next straight.

Of course, that description of how I take a corner sounds fairly simple when you break it down step-by-step. But of course when you try to do it at racing speeds, it's actually quite difficult. It took me a lot of practice to get the timing right for braking, corner entry, releasing brakes, hitting the apex, applying throttle to unlock the diff, exiting the corner, and smoothly applying power to accelerate away. But then that's what makes GPL so good - you have to learn and practice, and it makes it very rewarding when things start to go right!

Now let's get back to your question about different ramp angles. My setup for the 1967 Lotus 49 (original 1967 cars, not the 1967x mod) has diff settings of 70/40/2, which gives 5% power locking and 11% coast locking, and this fits in with my description above. There is another group of diff settings which would give almost the same locking: 75/65/4. The higher ramp angles reduce the locking, while doubling the clutches to 4 increases it again. These settings give 6% power locking and 11% coast locking.

Now I can't say definitely what the difference would be between those two diff setups as I've never tried 75/65/4. (Once I found 70/40/2 I settled with that and didn't experiment much further.) However, I think I can give an educated guess as to what should happen in theory. The "Four-Wheel Drift" book says that lower ramp angles with greater locking should give a quicker transition between locking states. Therefore 70/40/2 should give a quicker transition than 75/65/4.

So what effect does a quicker transition produce? Basically it means that the car will be a little less stable with 70/40/2 than with 75/65/4. Why would you want less stability in the corners? Less stability (in moderation) means more manoeuvrability, more ability to point the nose of the car. And with the diff states being controlled by the on/off throttle setting, that means you can use the throttle to lock/unlock the diff, and thus control the turning of the car in addition to the steering.

[Edit: The above paragraphs appear to be incorrect, see Lee's post #6 further down this thread. Different ramp angles don't seem to have different transition speeds, so it's only the locking percentages which affect the handling of the car. That simplifies things! ]

As I mentioned before, if I'm a bit too fast at the apex and in danger of understeering wide on the exit of the corner, I can use a 'blip' on the throttle to unstick the rear end and induce 'power-on oversteer' to turn the car towards the straight. It's a great feeling when you can do it successfully.

I hope that has given you some help. If there's anything that's not clear, feel free to post questions below and I'll check back as often as I can (maybe every 1 or 2 days) to answer them.

Of course, other GPL drivers may disagree with my methods, so you might get some different opinions!

TP, I usually stick with one diff locking setup for all tracks. It's nice to have consistent cornering behaviour so you can get to know the feel of the car. That's probably not ideal, but I enjoy driving as many different GPL tracks as possible, with all the different GPL mods and cars. I simply don't have time to customize every setting on the cars to suit each circuit individually.

The way I work is that I start off creating a baseline setup for each car, which is comfortable for my style of driving, and which allows the car to reach its maximum speed (not the maximum speed for a particular circuit, the cars maximum speed). My preferred suspension setup is pretty stiff, as I like a responsive car. This also has the advantage that on bumpy tracks (like Nurburgring) the car doesn't bottom out over the jumps and bumps. (The big dip at Fuchsrohre, between Aremberg and Adenauer Forst is the best place to test this if you need to check it.) I usually develop my setups at Monza and Bloodbath.

Monza is used to test cornering behaviour. Curva Grande and Ascari are great for checking high-speed cornering and drifting, and to check the behaviour of the diff/car under power and coast conditions. The Lesmos are great for testing braking behaviour, normal medium-speed cornering, and acceleration. Parabolica is good for testing hard braking, slow cornering (on the entry), and acceleration during cornering (on the exit).

I use Bloodbath to set my gear ratios. First gear is set to give me a good start from standstill. Top gear is set to allow the car to reach its top speed. And the other gears are spaced out between to stay within a good rev range, to give good acceleration, and to give a good transition between gears with the smallest gap possible.

Once all of that is done (and it takes a bit of time to test everything, switching back and forth between the two tracks a few times), I have my high-speed baseline setup that I can use at all high-speed tracks.

Then, I take the car to Monaco, and I adjust the diff ratio (but not the locking), and the gear ratios so that the car drives and accelerates well at one of GPL's slowest tracks. That gives me an alternate setup for slower tracks.

And then I go out and enjoy all of GPL's circuits, picking either the low-speed or high-speed setup to suit the track.

Some cars also benefit from having a medium-speed setup to get the best out of them. For example, the '67 Lotus can't use the low-speed gearing at Silverstone as it's just too slow. But when I use the high-speed gearing, I get reasonable lap times, but I only just get into fifth gear at the end of Hangar Straight, so top gear is wasted. To fix this, I adjust top gear to a medium value (so I can use it halfway along Hangar Straight, and also on the approach to Woodcote), and then space out gears 2, 3, and 4 as before to arrive at a medium-speed setup.

It's quite likely that having a looser diff at tight tracks like Monaco might help the car to turn just a little faster. But gearing is definitely the main factor which affects speed at different types of track, so I just concentrate on getting that right. As I mentioned before, I don't really have time to tinker with every little thing!

AFAIK in the code, there is no difference between how the car handles if the locking percentages are the same regardless of what ramp and clutch values are used.

That's interesting. That would definitely simplify the setup development process. All you have to do is check GPL Setup Manager to find the most comfortable power/coast percentages, and you're done.

Lee200, on Aug 18 2018 - 07:20 AM, said:

The only other point I'd make is that the coast locking percent is only applied if the throttle is completely off. Any other time, the power locking percent is used.

Yes, it's good to emphasise this. Some people use left-foot braking, and keep a small amount of pressure on the throttle all the way through a corner with their right foot. If you do that, you are permanently using the power locking side of the diff, you are never using the coast locking value, and there won't be any transition between the two values. You'll effectively have constant diff locking all the way around the lap.

The most important thing to a driver is driving and how to best apply the diff's capability.
So thanks Saiph for focusing on that.

BTW thanks again RF for that secret insight a few years ago, (!!!), regarding setting diff to fix speed of car's rotation when going into coast and back on power.

Duh! Why, I asked back then, had no one ever told us that before? Ever since, when reading about differentials or driving, if some expert does not mention that their authority falls off instantly.

Beginning to understand how tire tech so greatly influenced racing.
Tire tech changed the racing line.
In turn, tire innovations changed how drivers use the diff.

Big skinny tires had really big slip angles, (15degs), that made drifting with precision at high speed a delicate art.

The only way to get the rear out near the big slip angle optimum was if the diff was fast and loose enough in coast to allow it, then subtle and firm enough to get full power back on without passing the uncertain break-away limit.

Just a little false apex powerslide at 160k.

As tires got smaller, wider, softer, and stickier, optimum slip angles decreased steadily until now there is almost no slip angle, (<3degs), so no big slip powersliding going on anymore and just guessing, tighter faster diffs.

Millennium, Race Engineer, SA65, and Setup Manager, all I think allow various diff settings depending on the season/mod?

Wow, far more information than I was expecting. Really appreciate the explanation Saiph, and not a long read at all.

It's interesting what Lee says about coast locking only being applied if the throttle is completely off.

When I moved from an old 2 pedal and paddles set up to a shifter and clutch a few years ago I found the transition a lot harder than I expected. (But couldn't go back now). When I made the change I also moved to right foot braking as well and from what you are saying a lot of my issues were probably because I was unconsciously balancing the car using the throttle with my right foot while braking with my left. whereas with the 3 pedals I'm off throttle more and using the brakes for what they were intended so bringing the coast side of the diff more into play. it would certainly explain why I went from set up's with a 60 coast side all the was back to 20 before I was able to move back towards what I thought I was already good at.

Percent Locking/Anti Slip EffectivenessEarly LSDs (such as the Porsche ZF units) would not list the ramp angle in their specification. Rather they would use a percentage value to indicate the “percentage of anti slip”. So a “40% LSD” would have more slip than an “80% LSD”. Higher numbers equates to lower slip levels (or higher locking). This nomenclature was easier to use especially in early differentials that were symmetric 2 way designs.

Early Porsche ZF LSDs used 30 degree symmetric ramps. By varying the number of friction disks in the clutch pack, you could either create a 40% (2 friction disks), or a 80% (4 friction disks). Later Porsche LSDs used asymmetric designs, but they still used percentage values instead of ramp angles. In general a shallow ramp angle (smaller angle value) will result in a larger locking percentage and visa versa. So for example a 30 degree ramp may result in 80% locking and a 90 degree ramp may result in 0 % locking.

Most LSD manufactures list percent locking/anti-slip values instead of ramp angles as ramp angle is just part of the equation that defines the level of anti-slip.

How does anything about percents relate to driver priorities of extent of car's rotation in coast and speed of rotation?
Even tho percents are the same, maybe depending on car setup same extent of rotation, can speed of rotation be different?

Just thinking something has to be different, even if a little almost undetectable bit.

Going to RF braking took me years to catch up to my LF braking times. Which are all my GPL rank times. As a LF braker I used 45/85/1 and was never off the throttle completely. A very unrealistic way to drive 67 F1 cars and it always bothered me.

Now as a RF braker I use a diff of 85/45/5 and BB of 53%. I'm faster than i've ever been and it feels natural and real to how they drove in 67. I was surprised how long it took to get my old speed back.

Very illuminating thread this one. Saiph have you got the page no in 4WDrift that refers to locking times?

It's in the section called "What's the Diff?". In my book it's the second paragraph, just under the picture of the GPL gear ratios page, on page 53.

"Like almost everything else in this chapter, the devil is in the transition from one "state" to the another; in this case, from an "open" diff to a "locked" one. It isn't "digital", all on or off; it's a gradual transition. Just how gradual depends on two settings: the Ramp Angle (there are two, actually; one for power-on, one for power-off) and the number of Clutches. The Ramp Angles determine how abruptly the clutch-pack is engaged (the lower the number, the faster), and the number of clutches determines how tightly the rear wheels are locked together (the lower the number, the more loosely)."